Disposable absorbent garments such as infant diapers or training pants, adult incontinence products and other such products typically were constructed with a moisture-impervious outer backing sheet, a moisture-pervious body-contacting inner liner sheet, and a moisture-absorbent composite or core sandwiched between the liner and backing sheets. Much effort has been expended to find cost-effective materials for absorbent composites that display favorable liquid absorbency and retention. Superabsorbent materials in the form of granules, beads, fibers, bits of film, globules, etc., have been favored for such purposes. Such superabsorbent materials generally are polymeric gelling materials that are capable of absorbing and retaining even under moderate pressure large quantities of liquid, such as water and body wastes, relative to their own weight.
The superabsorbent material generally is a water-insoluble but water-swellable polymeric substance capable of absorbing water in an amount which is at least ten times the weight of the substance in its dry form. In one type of superabsorbent material, the particles or fibers may be described chemically as having a back bone of natural or synthetic polymers with hydrophilic groups or polymers containing hydrophilic groups being chemically bonded to the back bone or in intimate admixture therewith. Included in this class of materials are such modified polymers as sodium neutralized cross-linked polyacrylates and polysaccharides including, for example, cellulose and starch and regenerated cellulose which are modified to be carboxylated, phosphonoalkylated, sulphoxylated or phosphorylated, causing the superabsorbent polymer (SAP) to be highly hydrophilic. Such modified polymers may also be cross-linked to reduce their water-solubility.
The ability of a superabsorbent material to absorb liquid typically is dependent upon the form, position, and/or manner in which particles of the superabsorbent material are incorporated into the absorbent composite. Whenever a particle of the superabsorbent material and absorbent composite is wetted, it swells and forms a gel. Gel formation can block liquid transmission into the interior of the absorbent composite, a phenomenon called “gel blocking.” Gel blocking prevents liquid from rapidly diffusing or wicking past the “blocking” particles (e.g., those particles that have swelled and touched an adjacent swelled particle), causing portions of a partially hydrated core to become inaccessible to multiple doses of urine. Further absorption of liquid by the absorbent core must then take place via a diffusion process. This is typically much slower than the rate at which liquid is applied to the core. Gel blocking often leads to leakage from the absorbent article well before all of the absorbent material in the core is fully saturated.
Despite the incidence of gel blocking, superabsorbent materials are commonly incorporated into absorbent cores because they absorb and retain large quantities of liquid, even under load. However, in order for superabsorbent materials to function, the liquid being absorbed in the absorbent structure must be transported to unsaturated superabsorbent material. In other words, the superabsorbent material must be placed in a position to be contacted by liquid. Furthermore, as the superabsorbent material absorbs the liquid it must be allowed to swell. If the superabsorbent material is prevented from swelling, it will cease absorbing liquids.
Adequate absorbency of liquid by the absorbent core at the point of initial liquid contact and rapid distribution of liquid away from this point is desirable to ensure that the absorbent core has sufficient capacity to absorb subsequently deposited liquids. Previously known absorbent cores have thus attempted to absorb quickly and distribute large quantities of liquids throughout the absorbent core while minimizing gel blocking during absorption of multiple doses of liquid.
In general, some of the important performance attributes of an absorbent core of a diaper (or any other absorbent garment) are functional capacity, rate of absorption, core stability in use, AUL of SAP, ratio of fibrous material to SAP, the type and basis weight of glue or tackifying agent used to adhere the SAP to the fibrous material or tissue wrapping, and the basis weight of the core.
Absorption under load or AUL is a measure of functional capacity and the rate at which that absorption occurs. U.S. Pat. No. 5,147,343, the disclosure of which is incorporated herein by reference in its entirety, discloses that AUL is the ability of a superabsorbent material to swell under an applied force. Specifically, AUL is expressed as the amount (in millimeters) of an aqueous solution (0.9% weight percent sodium chloride) that a superabsorbent material can absorb per gram in one hour under a load of 21,000 dynes per square centimeter. AUL is believed to be a function of both SAP basis weight (mass per unit area) and the composition of SAP used in the composite.
It is known to provide absorbent composites comprised of, for example, upper and lower layers, and a central absorbent layer containing from 50% to 95% by weight SAP. U.S. Pat. No. 6,068,620, the disclosure of which is incorporated herein by reference in its entirety, discloses that the upper and lower layers are preferably comprised of tissue, airlaid fluff pulp or synthetic non-woven fibrous layers. The upper and lower layers are said to assist in maintaining the integrity of the core, the multi-layer layered arrangement is said to minimize gel blocking, and the multi-layer core can be folded in various configurations.
It also is known to provide absorbent cores comprised of differing materials in an attempt to maximize comfort and efficiency of the core, and to provide areas having varying degrees of absorbency. U.S. Pat. No. 5,849,002, the disclosure of which is incorporated by reference herein in its entirety, discloses absorbent cores having three zones: (i) one zone for receiving fluids; (ii) one zone for distributing and storing fluids; and (iii) one zone for preventing leakage. U.S. Pat. No. 5,853,402, the disclosure of which is incorporated by reference herein in its entirety, discloses composite absorbent cores comprising at least an absorbent material and a porous resilient material. Other composite, zoned, or multi-component cores are disclosed in, for example, U.S. Pat. Nos. 5,681,300 (blended absorbent core), 5,882,464 (crimping to join two absorbent structures), 5,891,120 (varying SAP concentration throughout core), 5,425,725 and 5,938,650 (multiple fiber free SAP pockets in core), and 5,922,165 (method of joining outer layers with absorbent core disposed between the outer layers). The respective disclosures of each of these documents are incorporated by reference herein in their entirety.